A new selective medium containing pentachloronitrobenzene and
2-ami nobutane - the PAB medium - was developed for use with the
soi1-di1ution plate method for the enumeration of fungal propagules
of F. solccni var. coeruleum and F. sulphureum in field soil. These
fungi cause a dry rotting of potatoes in storage. The efficiency of
the PAB medium in the measurement of levels of soil contamination was
compared with that of other methods. Also described is the PM70
medium, suitable for the isolation of a number of pathogens, including P. exigua var. foveata from diseased tuber tissue.
The incidence of dry rot after grading was related logarithmically to the number of F. solccni var. coeruleum propagules in progeny
tubersphere soil. Highest levels of progeny tuber contamination with
F. solccni var. coeruleum were associated with the planting of infected
seed. Planting of contaminated seed sometimes gave high levels of
transmission, possibly because dry rot developed after planting.
Propagule production by infected seed varied between seasons and may
be related to soil temperature. Tuber factors, eg variety and seed
size, also influenced propagule production. Of the seed treatments
tested, only thiabendazole reduced consistently the transmission of
F. solccni var. coeruleum.
Levels of soil contamination increased during the growing season
but removal of the seed tuber prevented further increase. Highest
numbers of propagules were in a 5 cm diam. zone of soil surrounding
the seed and spread of propagules was mainly lateral and downwards
forming a decreasing gradient of inoculum with increasing distance
from the seed tuber. Propagule distribution on progeny tubers
followed the same pattern but harvesting by elevator digger disturbed
the soil inoculum, making all progeny tubers highly contaminated
F. solani var. coeruleum survived a 6 year rotation in field
soil and this soil-borne inoculum is possibly important in the
re-contamination of clean seed stocks.
Most varieties, of those tested, were resistant to infection by
F. solani var. coeruleum in November but susceptible by February.
Tubers were more susceptible if incubated initially at 4°C rather
than at 15°C.
Transmission of F. solani var. coeruleum was compared with that
of F. sulphureum. Propagules of F. solani var. coeruleum were produced
in cavities in the tuber and in pustules on the tuber surface but
F. sulphureum showed little surface sporulation. Thus thiabendazole,
which inhibits surface sporulation was inconsistent in reducing transmission of F. sulphureum. Moreover, F. sulphureum sporulated on stems
growing from infected seed tubers in the field.
F. sulphureum infected seed usually produced less inoculum than
did F. solani var. coeruleum but in one season the reverse was true
and was possibly related to high soil temperatures. F. sulphureum
does not appear to survive in field soil as well as F. solani var.
coeruleum. Although F. sulphureum seems less well adapted than
F. solani var. coeruleum for propagule transmission none of the
varieties tested was resistant to infection by F. sulphureum.